A gel can reduce a flash’s color rendering somewhat, but usually not by much. A bare flash is designed to approximate daylight fairly well, so it starts with good color rendering. Adding a gel changes the spectrum by attenuating some wavelengths more than others, which can lower CRI because some colors will be rendered less accurately.

That said, typical flash gels do not create severe “holes” in the spectrum. They usually just favor some wavelengths slightly over others. Light sources with genuinely poor CRI—such as sources missing large parts of the visible spectrum—tend to be much worse for skin tones and color accuracy than a gelled flash.

So in practice, a gelled flash will often still render color better than many low-CRI decorative lights, while also matching the scene’s warm look more closely. It won’t be perfect, and exact color accuracy may still be limited, but a gel generally does not devastate flash color rendering the way a poor-quality light source can.